The mitochondrial DNA (mtDNA) of Japanese patients suffering from the syndrome of mitochondrial myopathy, encephalopathy, lactic acidosis and stroke-like episodes (MELAS) exhibits a specific heteroplasmic A----G transition in the tRNA(Leu) at position 3243. In this study, we investigated mtDNA from skeletal muscle, cardiac muscle, brain, liver, diaphragm, fibroblasts and blood cells of four Caucasians with MELAS, one younger healthy sister of two MELAS patients, and eleven controls. We found that 1) the mutation was present in all investigated tissues of Caucasians with MELAS but not in controls, 2) within a single patient, the tissue-specific variation of the copy number of mutated mtDNA covered the same range as in the skeletal muscle of different patients, 3) the mutation was also present in the blood cells of the healthy sister of two MELAS siblings.